Method for producing composition containing low molecular weight polytetrafluoroethylene

ABSTRACT

(II) carrying out fluorination treatment on the composition containing low molecular weight polytetrafluoroethylene obtained above.

TECHNICAL FIELD

The present invention relates to a method for producing a compositioncontaining low molecular weight polytetrafluoroethylene.

BACKGROUND ART

Since low molecular weight polytetrafluoroethylene (sometimes referredto as “low molecular weight PTFE” hereinafter) not only has excellentchemical stability and extremely low surface energy but also is unlikelyto suffer fibrillation, it has been used for producing plastics, inks,cosmetics, coating materials, greases, etc. as an additive for enhancinglubricity or texture of a coating surface (e.g., Patent Literature 1).One of the methods for producing a composition containing such lowmolecular weight PTFE is a method of irradiating high molecular weightpolytetrafluoroethylene (sometimes referred to as “PTFE” hereinafter)with ionizing radiation (e.g., Patent Literature 2).

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open No. 10-147617-   Patent Literature 2: Japanese Patent Publication No. 47-19609

SUMMARY OF INVENTION Technical Problem

In such a composition containing low molecular weight PTFE as in PatentLiterature 2, a low molecular weight fluorine-containing compound havinga functional group and its derivative (sometimes referred to as “PFC”hereinafter), such as perfluorooctanoic acid (sometimes referred to as“PFOA” hereinafter) or its salt, can be contained. As a result ofearnest studies by the present inventor, it has been found that PFCtypified by such PFOA as above can be produced by irradiation withionizing radiation.

It is an object of the present invention to obtain a compositioncontaining low molecular weight PTFE, which has been reduced in acontent of PFC.

Solution to Problem

According to a first aspect of the present invention, provided is amethod for producing a composition containing low molecular weight PTFE,comprising:

(I) irradiating a composition containing PTFE with ionizing radiation toobtain a composition containing low molecular weight PTFE having a meltviscosity at 380° C. in the range of 1.0×10² to 7.0×10⁵ Pa·s; and

(II) carrying out fluorination treatment on the composition containinglow molecular weight PTFE obtained above.

Advantageous Effects of Invention

According to the present invention, a method suitable for producing acomposition containing low molecular weight PTFE, which has been reducedin a content of PFC, can be obtained.

DESCRIPTION OF EMBODIMENTS

The method for producing a composition containing low molecular weightPTFE of the present invention comprises:

(I) irradiating a composition containing PTFE with ionizing radiation toobtain a composition containing low molecular weight PTFE having a meltviscosity at 380° C. in the range of 1.0×10² to 7.0×10⁵ Pa·s (sometimesreferred to as “step (I)” hereinafter); and

(II) carrying out fluorination treatment on the composition containinglow molecular weight PTFE obtained above (sometimes referred to as “step(II)” hereinafter).

The step (I) will be described hereinafter.

The PTFE can be one obtained by a polymerization method that can beusually carried out, such as emulsion polymerization or suspensionpolymerization.

The PTFE may be one having, as a constituent unit derived from a monomerin a molecular structure, only a constituent unit derived fromtetrafluoroethylene (TFE), or a constituent unit derived from a monomerhaving a structure other than TFE and a constituent unit derived fromTFE (sometimes referred to as “modified PTFE” hereinafter).

The monomer having a structure other than TFE is not limited as long asit is copolymerizable with TFE, and a monomer usually used (e.g.,perfluoroolefin such as hexafluoropropylene [HFP]; chlorofluoroolefinsuch as chlorotrifluoroethylene [CTFE]; hydrogen atom-containingfluoroolefin such as trifluoroethylene or vinylidene fluoride [VDF];perfluorovinyl ether; perfluoroalkylethylene; ethylene) can be used.Only one of the monomers having a structure other than TFE may be used,or a plurality of them may be used.

Examples of the perfluorovinyl ether include, but not limited to, anunsaturated perfluoro compound represented by the following generalformula (1):

CF₂═CF—ORf   (1)

wherein Rf represents a perfluoro organic group. In the presentspecification, the “perfluoro organic group” means an organic group inwhich hydrogen atoms bonded to carbon atoms are all substituted byfluorine atoms. Examples of the perfluoro organic group include aperfluoroalkyl group and a perfluoro(alkoxyalkyl) group. The perfluoroorganic group may have an oxygen atom that forms an ether linkage.

In one embodiment, the perfluorovinyl ether is, for example, aperfluoroalkyl vinyl ether (PAVE), wherein Rf is a perfluoroalkyl grouphaving 1 to 10 carbon atoms (preferably 1 to 5 carbon atoms) in theabove general formula (1).

Examples of the perfluoroalkyl group in the PAVE include aperfluoromethyl group, a perfluoroethyl group, a perfluoropropyl group,a perfluorobutyl group, a perfluoropentyl group and a perfluorohexylgroup. A preferred perfluoroalkyl group is, for example, aperfluoropropyl group.

Preferred PAVE is perfluoropropyl vinyl ether (PPVE) in which theperfluoroalkyl group is a perfluoropropyl group.

In another embodiment, the perfluorovinyl ether is, for example, in thegeneral formula (1), one in which Rf is a perfluoro(alkoxyalkyl) grouphaving 4 to 9 carbon atoms;

one in which Rf is a group represented by the following formula:

wherein m is 0 or an integer of 1 to 4; or

one in which Rf is a group represented by the following formula:

wherein n is an integer of 1 to 4.

Examples of the perfluoroalkylethylene include, but not limited to,(perfluorobutyl)ethylene (PFBE), (perfluorohexyl)ethylene and(perfluorooctyl)ethylene.

The monomer having a structure other than TFE is preferably at least oneselected from the group consisting of HFP, CTFE, VDF, PPVE, PFBE andethylene, and is more preferably at least one selected from the groupconsisting of HFP and CTFE.

In the modified PTFE, the constituent unit derived from the monomerhaving a structure other than TFE is preferably contained in an amountin the range of 0.001 to 1 mass %. In the modified PTFE, the constituentunit derived from the monomer having a structure other than TFE is morepreferably contained in an amount of 0.01 mass % or more. In themodified PTFE, the constituent unit derived from the monomer having astructure other than TFE is more preferably contained in an amount of0.5 mass % or less, and is still more preferably contained in an amountof 0.1 mass % or less. The content of the constituent unit derived fromthe monomer having a structure other than TFE can be determined by aknown method such as Fourier transform infrared spectroscopy (FT-IR).

A standard specific gravity (SSG) of the PTFE is preferably in the rangeof 2.130 to 2.230. The SSG is a value measured in accordance with ASTM D4894 when the PTFE is obtained by suspension polymerization, and is avalue measured in accordance with ASTM D 4895 when the PTFE is obtainedby emulsion polymerization. SSG or a melt viscosity can be usually usedas an indication of a molecular weight of PTFE. PTFE having such SSG asabove has an extremely high melt viscosity, and it is difficult toaccurately measure the melt viscosity. Accordingly, in PTFE having suchSSG as above, the SSG is usually used as an indication of the molecularweight.

A melting point of the PTFE is preferably in the range of 324° C. to336° C. The melting point is a value measured by using, for example, adifferential scanning calorimeter (DSC). Specifically, the melting pointcan be a minimum point of heat of fusion, which is obtained by using aDSC having been subjected to temperature calibration in advance usingindium and lead as standard samples and increasing the temperature at10° C./min in the temperature region of 250° C. to 380° C. in an airstream at 200 ml/min.

The composition containing PTFE is preferably substantially composed ofPTFE. The expression “substantially composed of PTFE” means that PTFEcan be contained in an amount of 90 mass parts or more, specifically inan amount of 95 mass parts or more, based on 100 mass parts of thecomposition. The upper limit of the content of PTFE based on 100 massparts of the composition is not limited, but it can be, for example, 100mass parts or less, specifically 98 mass parts or less.

The composition containing PTFE may be one containing PFOA or its salt,or may be one substantially free from PFOA or its salt. Here, theexpression “substantially free from” means that the amount of PFOA, interms of mass, based on PTFE is less than 25 ppb, preferably 15 ppb orless, more preferably 5 ppb or less, particularly preferably less thanthe detection limit. The expression “less than the detection limit”means, for example, less than 5 ppb. In the present specification, theabove content indicates a total amount of PFOA and its salt, unlessotherwise noted.

The composition containing PTFE is preferably substantially free fromPFOA or its salt. Here, the expression “substantially free from” has thesame meaning as above. Even when the composition containing PTFE andsubstantially free from PFOA or its salt is used as in the presentembodiment, PFOA or its salt can be formed by irradiating the PTFE withionizing radiation in the step (I).

Examples of the salt of PFOA include an ammonium salt, a sodium salt anda potassium salt of PFOA.

The content of the PFOA and its salt can be measured by using liquidchromatography. More specifically, the PFOA and its salt are extractedfrom the composition containing PTFE using a solvent, and the amount ofthe PFOA and its salt contained in the solvent after extraction can bemeasured by using liquid chromatography.

As the composition containing PTFE, one containing PFC may be used, orone substantially free from PFC may be used. Here, the expression“substantially free from” means that the amount of PFC, in terms ofmass, based on PTFE is less than 25 ppb, preferably 15 ppb or less, morepreferably 5 ppb or less, particularly preferably less than thedetection limit. The expression “less than the detection limit” means,for example, less than 5 ppb.

The composition containing PTFE is preferably substantially free fromPFC. Even when the composition containing PTFE and substantially freefrom PFC is used as in the present embodiment, PFC can be formed byirradiating the PTFE with ionizing radiation in the step (I). Here, theexpression “substantially free from” has the same meaning as above. Inthe present specification, when PFCs of a plurality of structures ispresent, the above content indicates a total amount of PFCs of aplurality of structures, unless otherwise noted.

The content of the PFC can be measured by using, for example, liquidchromatography. More specifically, from the composition containing PTFE,the PFC is extracted using a solvent, and the amount of the PFCcontained in the solvent after extraction can be measured by usingliquid chromatography.

In the present specification, “PFC” means a low molecular weightfluorine-containing compound having a functional group and itsderivative. The functional group preferably exists at the molecular endof the low molecular weight fluorine-containing compound. Examples ofthe functional group include a carboxyl group and a sulfonic acid group.Examples of the derivative include salts. Examples of the salt includean ammonium salt, a sodium salt and a potassium salt.

The low molecular weight fluorine-containing compound having afunctional group is specifically, for example, an acid having a lowmolecular weight fluorine-containing carbon chain. Thefluorine-containing carbon chain indicates a carbon chain in which oneor more of hydrogen atoms bonded to carbon atoms are substituted byfluorine atoms. Examples of the acid include carboxylic acid andsulfonic acid.

The low molecular weight fluorine-containing compound is, for example, afluorine-containing compound in which 6 to 14 carbon atoms are containedin a fluorine-containing carbon chain, specifically an acid having afluorine-containing carbon chain having 6 to 14 carbon atoms.

That is to say, the PFC is specifically, for example, an acid having afluorine-containing carbon chain having 6 to 14 carbon atoms or itsderivative.

Examples of the shape of the composition containing PTFE include, butnot limited to, a powder, a molded body, a scrap, a fragment that can beproduced during formation of a molded body, a cutting chip producedduring cutting of a molded body, and a preform.

In one embodiment, the shape of the composition containing PTFE ispowder. The present embodiment is advantageous from the viewpoint thatthe resin component containing PTFE can be uniformly irradiated withionizing radiation. According to the present embodiment, a powderycomposition containing low molecular weight PTFE can be easily obtained.

In one embodiment, the composition containing PTFE is a molded body. Inthe present embodiment, the composition containing PTFE can be onehaving been subjected to a molding step of heating the compositioncontaining PTFE to a primary melting point or higher. That is to say,the production method of the present invention can further include,prior to the step (I), a step of heating the composition containing PTFEto a primary melting point of PTFE or higher to form a molded body ofthe composition containing PTFE.

The primary melting point can be measured by using DSC. Specifically,the primary melting point is a maximum peak temperature of anendothermic curve appearing on a crystal melting curve, and theendothermic curve is obtained by heating uncalcined PTFE under theconditions of a temperature-increasing rate of 10° C./min. The primarymelting point can be usually a temperature of 320° C. or higher. Theuncalcined PTFE indicates PTFE free from heat history of being heated toa temperature of the primary melting point or higher.

The shape of the molded body is not limited. Examples of the shape ofthe molded body include tape-like, sheet-like, rod-like, tubular, andfibrous shapes.

A specific gravity of the molded body is preferably 1.0 g/cm³ or more,more preferably 1.5 g/cm³ or more, and is preferably 2.5 g/cm³ or less.The specific gravity can be measured by an underwater replacementmethod.

In one embodiment, the composition containing PTFE is a scrap, afragment that can be produced during formation of a molded body, or acutting chip produced during cutting of a molded body.

In one embodiment, the composition containing PTFE is a preform. Here,the preform refers to a molded body that is obtained by compressingpowdery PTFE and has not been subjected to a calcining step (has notbeen heated to a temperature of the primary melting point or higher).

Irradiation with ionizing radiation can be carried out by aconventionally known method and under the conventionally knownconditions. By the irradiation with ionizing radiation, the PTFEcontained in the composition containing PTFE is decreased in molecularweight and becomes low molecular weight PTFE. By the irradiation withionizing radiation, PFC such as PFOA or its salt can be produced.

Examples of the ionizing radiation include electron ray, γ-ray, X-ray,neutron beam and high energy ion. The ionizing radiation is preferablyelectron ray or γ-ray.

The irradiation with ionizing radiation is not limited, but can becarried out in, for example, air, a hydrocarbon-based gas, water, or asolvent.

In one embodiment, the irradiation with ionizing radiation can becarried out in air. The present embodiment is preferable from theviewpoint of reduction of cost.

An irradiation dose of the ionizing radiation is preferably in the rangeof 1 to 2500 kGy. The irradiation dose of the ionizing radiation is morepreferably 1000 kGy or less, still more preferably 750 kGy or less. Theirradiation dose of the ionizing radiation is more preferably 10 kGy ormore, particularly preferably 50 kGy or more.

An irradiation temperature of the ionizing radiation is, for example, 0°C. or higher. The upper limit of the irradiation temperature of theionizing radiation is not limited as long as it is a melting point ofPTFE or lower. From the viewpoint of suppression of crosslinking of PTFEmolecular chain, the irradiation temperature of the ionizing radiationis preferably 320° C. or lower, more preferably 300° C. or lower, stillmore preferably 260° C. or lower. From the viewpoint of reduction ofproduction cost, irradiation with ionizing radiation can be carried outat 15° C. (e.g., 19° C.) to 60° C.

The resin component contained in the composition obtained in the step(I) is preferably substantially composed of low molecular weight PTFE.The expression “substantially composed of low molecular weight PTFE”means that the low molecular weight PTFE can be contained in an amountof, for example, 90 mass parts or more, specifically 95 mass parts ormore, based on 100 mass parts of the composition. The upper limit of thelow molecular weight PTFE based on 100 mass parts of the composition isnot limited, but it can be, for example, 100 mass parts or less,specifically 98 mass parts or less.

The shape of the composition obtained in the step (I) is not limited.

In one embodiment, the shape of the composition containing low molecularweight PTFE obtained in the step (I) is powder. This composition isadvantageous from the viewpoint of ease of treatment in or after thestep (II).

In the above embodiment, an average particle size of the low molecularweight PTFE in the composition obtained in the step (I) is preferably1000 μm or less, more preferably 300 μm or less, still more preferably100 μm or less. The lower limit of the average particle size of the lowmolecular weight PTFE is not limited, but it is, for example, more than1 μm. Since the low molecular weight PTFE has such an average particlesize, the diameters of particles contained in the composition can becomerelatively small.

The average particle size of the low molecular weight PTFE can bemeasured by using a laser diffraction particle size distributionmeasurement apparatus. Specifically, the average particle size is takento be equal to a particle size corresponding to 50% (based on volume) ofparticle size distribution integration obtained by carrying outmeasurement at a dispersion pressure of 1.0 bar without using a cascade.The laser diffraction particle size distribution measurement apparatusmay be, for example, laser diffraction particle size distributionmeasurement apparatus manufactured by JEOL Ltd. (trade name: HELOS &RODOS).

A melt viscosity of the low molecular weight PTFE at 380° C. is in therange of 1.0×10² to 7.0×10⁵ Pa·s. A more preferred lower limit of themelt viscosity is 1.5×10³ Pa·s or more. A more preferred upper limit ofthe melt viscosity is 3.0×10⁵ Pa·s or less. In the present invention,the “low molecular weight PTFE” means PTFE having a melt viscosity inthe above range. In the case of low molecular weight PTFE having such amelt viscosity as above, it is difficult to form a molded body for usein the measurement of the aforesaid standard specific gravity (SSG), andit is difficult to measure accurate SSG of the low molecular weightPTFE. Accordingly, a melt viscosity is usually used as an indication ofa molecular weight of low molecular weight PTFE.

In accordance with ASTM D 1238, the melt viscosity can be measured using2 g of a sample having been heated in advance at the measuringtemperature (380° C.) for 5 minutes and using a flow tester(manufactured by Shimadzu Corporation) and a die of 2ϕ-8 L whilemaintaining the sample at the above temperature under a load of 0.7 MPa.

The low molecular weight PTFE can be non-fibrillatable.

A melting point of the low molecular weight PTFE is preferably in therange of 324° C. to 336° C. This melting point can be measured using amethod described above as the method for measuring a melting point ofPTFE.

The step (II) will be described hereinafter.

The step (II) is a step of carrying out fluorination treatment on thecomposition containing low molecular weight PTFE obtained in the step(I). By carrying out the step (II), a composition containing lowmolecular weight PTFE and having a low content of PFC can be obtained.

Through the step (II), the composition containing low molecular weightPTFE obtained in the step (I), specifically the composition containingPFC, is fluorinated. Owing to the fluorination treatment in the step(II), a fluorine atom can be introduced into a functional group (e.g.,carboxyl group, sulfonic acid group) or its salt contained in PFC. Forexample, when the functional group is a carboxyl group, the carboxylgroup can be converted into —COOF, —COF, —F or the like, andsubsequently can be converted into —CF₃. By carrying out fluorinationtreatment in the step (II) as above, the concentration of PFC containedin the composition containing low molecular weight PTFE is reduced.

Although the content of PFC contained in the composition containing lowmolecular weight PTFE obtained in the step (I) is not limited, it is,for example, 25 mass ppb or more based on the low molecular weight PTFE.

Although the content of PFOA or its salt contained in the compositionobtained in the step (I) is not limited, it is, for example, 25 mass ppbor more based on the low molecular weight PTFE.

The fluorination treatment can be carried out by bringing thecomposition containing low molecular weight PTFE obtained in the step(I) and at least a fluorinating agent into contact with each other.

Examples of the fluorinating agent include, but not limited to,nucleophilic fluorinating agents, such as fluorine gas, HF, KF, CsF,Bu₄NF and 4-tert-butyl-2,6-dimethylphenylsulfur trifluoride; andelectrophilic fluorinating agents, such asN-fluoro-N′-(chloromethyl)triethylenediamine bis(tetrafluoroborate).From the viewpoints of economic efficiency and productivity, it ispreferable to use fluorine gas as the fluorinating agent.

In one embodiment, a diluent gas can be used together with thefluorinating agent in the fluorination treatment.

In the above embodiment, the concentration of the fluorinating agent(e.g., fluorine gas) based on the fluorinating agent and the diluent gasis not limited, but it is, for example, 0.01% or more, preferably 0.1%or more, more preferably 1% or more, in terms of volume ratio, based on100% of the total of the fluorinating agent and the diluent gas. Theupper limit of the concentration of the fluorinating agent (e.g.,fluorine gas) is not limited, but for the purpose of avoiding a dangerof heat generation of the apparatus itself or the like, it is preferably50% or less, more preferably 30% or less, in terms of volume ratio,based on 100% of the total of the fluorinating agent and the diluentgas.

The diluent gas is not limited as long as it is an inert gas, and forexample, helium gas, argon gas or nitrogen gas can be used. From theviewpoint of safe production and from the viewpoint of suppression ofproduction cost, the diluent gas is preferably nitrogen gas.

The contact of the composition containing low molecular weight PTFEobtained in the step (I) with at least the fluorinating agent (forexample, fluorinating agent and diluent gas, specifically fluorine gasand diluent gas) can be carried out by a constant flow method for thefluorinating agent (for example, fluorinating agent and diluent gas,specifically fluorine gas and diluent gas) or a batch method. Thecontact can be carried out by allowing at least the fluorinating agentto continuously flow in a container in which the composition containinglow molecular weight PTFE is placed, or can be carried out in such astate that the composition containing low molecular weight PTFE and atleast the fluorinating agent are sealed in a sealable container.

From the viewpoint of production stability, it is preferable to carryout the contact in such a state that the composition containing lowmolecular weight PTFE and at least the fluorinating agent are sealed ina sealable container.

In one embodiment, the contact can be carried out using a stainlesssteel sealable container.

The contact can be carried out in environment of, for example, 5° C. orhigher and 300° C. or lower.

In one embodiment, in order to enhance reactivity between thefluorinating agent and PFC, the contact is preferably carried out at 50°C. or higher, and is more preferably carried out at 100° C. or higher.From the viewpoint of production stability, the contact is preferablycarried out at 300° C. or lower, and is more preferably carried out at225° C. or lower.

The pressure for carrying out the contact is not limited. The contactmay be carried out under pressure, or may be carried out at normalpressure.

In one embodiment, the contact can be carried out at normal pressure(e.g., gauge pressure of 0 to 0.01 MPa). For example, the contact can becarried out by sealing the low molecular weight PTFE and at least thefluorinating agent in a stainless steel container in such a manner thatthe pressure in the container becomes normal pressure. The presentembodiment is particularly advantageous from the viewpoint of stabilityof production.

The step (II) can further include grinding treatment. Specifically, thestep (II) can include carrying out fluorination treatment and grindingtreatment on the composition containing low molecular weight PTFEobtained in the step (I).

In a preferred embodiment, in the step (II), grinding treatment iscarried out on the composition containing low molecular weight PTFEobtained in the step (I), and thereafter, fluorination treatment iscarried out on the ground composition. According to the presentembodiment, the content of PFC typified by PFOA or its salt can beeffectively reduced.

A method for the grinding treatment is not limited, but it is, forexample, a method of grinding using a grinder. Examples of the grinderinclude impact type grinders, such as a hammer mill, a pin mill and ajet mill, and mill type grinders, such as a cutter mill that performsgrinding by means of shear force produced by a rotary blade and an outerperipheral stator (stationary blade).

The temperature for carrying out the grinding treatment is preferably−200° C. or higher and lower than 50° C.

In one embodiment, the grinding is freeze grinding and can be usuallycarried out at a temperature in the range of −200° C. to −100° C. In thepresent embodiment, the temperature can be generally adjusted usingliquid nitrogen.

In one embodiment, the grinding is carried out at a temperature in thevicinity of room temperature (e.g., in the range of 10° C. to 50° C.).

In one embodiment, the grinding can be carried out at 10° C. or higherand lower than 50° C., is preferably carried out at a temperature in therange of 10° C. to 40° C., and is more preferably carried out at atemperature in the range of 10° C. to 30° C. The present embodiment ispreferable from the viewpoints of simplification of the step andreduction of cost required for the grinding.

The method for producing a composition containing low molecular weightPTFE of the present invention may further include classificationtreatment.

In the classification treatment, for example, airflow classification canbe used. Specifically, the classification treatment may be one in whichfine particles or fibrous particles are removed by airflowclassification and then coarse particles are further removed byclassification.

In the airflow classification, low molecular weight PTFE particles(e.g., ground particles) are fed to a column-like classification room bydecompressed air and dispersed by swirl airflow in the room, and thefine particles are classified by centrifugal force. The fine particlesare recovered with a cyclone and a bug filter from the center. In theclassification room, a cone in a conical shape or a rotating body suchas a rotor is installed in order to allow the ground particles and airto uniformly perform swirling motion.

When classification cones are used, adjustment of classification pointsis carried out by adjusting a flow rate of secondary air and a gapbetween the classification cones. When a rotor is used, a flow rate inthe classification room is adjusted by the number of revolutions of therotor.

Examples of the method for removing coarse particles include airflowclassification using a mesh, a vibrating screen and an ultrasonic sieve,and airflow classification is preferable.

In one embodiment, the classification treatment may be carried out priorto the step (I).

In one embodiment, the classification treatment is carried out on thecomposition having been subjected to the grinding treatment.

In a preferred embodiment, the step (II) is a step in which grindingtreatment is carried out on the composition containing low molecularweight PTFE obtained in the step (I), then classification treatment iscarried out, and thereafter, fluorination treatment is carried out toobtain a composition containing low molecular weight PTFE.

In another preferred embodiment, the step (II) is a step in whichgrinding treatment is carried out on the composition obtained in thestep (I), then fluorination treatment is carried out, and thereafter,classification treatment is carried out to obtain a compositioncontaining low molecular weight PTFE.

A content of PFOA and its salt contained in the composition containinglow molecular weight PTFE obtained in the present invention ispreferably less than 25 ppb, more preferably 15 ppb or less, still morepreferably less than 5 ppb, in terms of mass, based on the low molecularweight PTFE. The lower limit of the content of PFOA and its salt is notlimited, but it is, for example, less than the detection limit, morespecifically 0 ppb. According to the present invention, a compositioncontaining low molecular weight PTFE and having a low content of PFOAand its salt as described above can be obtained. The above contentindicates a total amount of PFOA and its salt. The salt of PFOA is aspreviously described.

The content of the PFOA and its salt can be measured by using liquidchromatography. More specifically, from the composition that is ameasuring object, the PFOA and its salt are extracted using a solvent,and the amount of the PFOA and its salt contained in the solvent afterextraction can be measured by using liquid chromatography.

A content of PFC contained in the composition containing low molecularweight PTFE obtained in the present invention is preferably 50 ppb orless, more preferably less than 25 ppb, still more preferably 15 ppb orless, particularly preferably 5 ppb or less, more preferably less than 5ppb, in terms of mass, based on the low molecular weight PTFE. The lowerlimit of the content of the PFC is not limited, but it is, for example,less than the detection limit, more specifically 0 ppb. According to thepresent invention, a composition containing low molecular weight PTFEand having a low content of PFC as described above can be obtained. WhenPFCs of a plurality of structures are present, the above contentindicates a total amount of PFCs of a plurality of structures. The PFCis as previously described.

The content of the PFC can be measured by using, for example, liquidchromatography. More specifically, from the composition that is ameasuring object, the PFC is extracted using a solvent, and the amountof the PFC contained in the solvent after extraction can be measured byusing liquid chromatography.

In one embodiment, the shape of the composition containing low molecularweight PTFE obtained by the production method of the present inventionis powder. A specific surface area of the powder of the presentembodiment is preferably in the range of 0.1 to 30.0 m²/g. The specificsurface area can be obtained by using a surface analyzer. Specifically,the specific surface area is measured by BET method using a mixed gasconsisting of 30% of nitrogen and 70% of helium as a carrier gas andusing liquid nitrogen as a refrigerant. The surface analyzer may be, forexample, BELSORP-mini II (trade name, manufactured by MicrotracBELCorp.). The specific surface area is more preferably 7.0 m²/g or more.

In one embodiment, the shape of the composition containing low molecularweight PTFE obtained by the production method of the present inventionis powder. A specific surface area of the powder of the presentembodiment is 0.1 m²/g or more and less than 7.0 m²/g. A method formeasuring the specific surface area is as described above. Such lowmolecular weight PTFE can be easily dispersed in a matrix material. Thematrix material may be a plastic, an ink, a coating material, or thelike.

The lower limit of the specific surface area of the powder(specifically, low molecular weight PTFE) of the present embodiment ispreferably 1.0 m²/g or more. The upper limit of the specific surfacearea of the powder of the present embodiment is preferably 5.0 m²/g orless, more preferably 3.0 m²/g or less.

In one embodiment, the shape of the composition containing low molecularweight PTFE obtained by the production method of the present inventionis powder. A specific surface area of the powder of the presentembodiment is 7.0 m²/g or more and 30.0 m²/g or less. A method formeasuring the specific surface area is as described above. In the caseof such low molecular weight PTFE, the sizes of particles dispersed in amatrix material are small, so that an effect of surface modification,such as an effect of enhancing texture of a coating surface, is high,and the oil absorption can be high. The matrix material may be an oil, agrease, a coating material, a plastic, or the like.

The lower limit of the specific surface area of the powder(specifically, low molecular weight PTFE) of the above embodiment ispreferably 8.0 m²/g or more. The upper limit of the specific surfacearea of the low molecular weight PTFE contained in the composition ofthe present embodiment is preferably 30.0 m²/g or less, more preferably20.0 m²/g or less.

In one embodiment, the shape of the composition containing low molecularweight PTFE obtained by the production method of the present inventionis powder. An average particle size of the powder of the presentembodiment is preferably in the range of 0.5 to 200 μm. The averageparticle size is more preferably 100 μm or less, still more preferably50 μm or less. The average particle size is, for example, 1.0 μm ormore. The low molecular weight PTFE obtained by the present inventionhas such an average particle size as above, and therefore, when it isadded to a coating material as, for example, an additive, it cancontribute to formation of a coating film having more excellent surfacesmoothness.

A method for measuring the average particle size of the compositioncontaining low molecular weight PTFE obtained by the production methodof the present invention is as described as the method for measuring theaverage particle size of the low molecular weight PTFE contained in thecomposition obtained in the step (I).

In a preferred embodiment, the composition containing low molecularweight PTFE obtained by the production method of the present inventionis in powder form.

The composition containing low molecular weight PTFE obtained by thepresent invention can be used after it is molded, when needed.

In one embodiment, the method of the present invention is a method forpurifying a composition containing low molecular weight PTFE,comprising:

irradiating a composition containing PTFE with ionizing radiation toobtain a composition containing low molecular weight PTFE having a meltviscosity at 380° C. in the range of 1.0×10² to 7.0×10⁵ Pa·s; and

carrying out fluorination treatment on the composition containing lowmolecular weight PTFE obtained above.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be preferably used asan additive for modifying a molding material, an ink, a cosmetic, acoating material, a grease, a member for office automation equipment, atoner or the like, an additive for a plating solution, or the like.Examples of the molding material include engineering plastics, such aspolyoxybenzoyl polyester, polyimide, polyamide, polyamideimide,polyacetal, polycarbonate and polyphenylene sulfide. The compositioncontaining low molecular weight PTFE can be preferably used particularlyas a thickening agent for grease.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be used as an additivefor a molding material. The composition can be preferably used, forexample, for the purpose of enhancing non-stickiness and slidingproperty of a copy roll or a photoreceptor; for the purpose of enhancingtexture of engineering plastic molded products, such as a surface layersheet of furniture, a dashboard of an automobile, and a cover of anappliance; for the purpose of enhancing lubricity or abrasion resistanceof machine parts that can cause mechanical friction, such as alight-load bearing, a gear, a cum, buttons of push-button telephone, aprojector, camera parts, and a sliding material; or as a processing aidfor engineering plastics, or the like.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be used for thepurpose of enhancing lubricity of a varnish or a paint, as an additivefor a coating material. The composition containing low molecular weightPTFE can be used for the purpose of enhancing lubricity of cosmeticssuch as foundation, as an additive for cosmetics.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be preferably usedalso for the purpose of enhancing oil-repellency or water-repellency ofa wax or the like and for the purpose of enhancing lubricity of athickening agent for grease, a grease or a toner.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be used also as anelectrode binder for a secondary battery or a fuel battery, a hardnessadjuster for an electrode binder, a water-repellent for an electrodesurface, or the like.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be used together witha lubricating oil to prepare a grease. Since the grease is characterizedby containing the powdery composition containing low molecular weightPTFE and a lubricating oil, the composition containing low molecularweight PTFE can be uniformly and stably dispersed in the lubricatingoil. The grease can be excellent in properties such as heat resistance,load-bearing property, electrical insulation property and lowhygroscopicity.

The lubricating oil (base oil) may be a mineral oil, or may be asynthetic oil. Examples of the lubricating oil (base oil) includeparaffinic or naphthenic mineral oils, and synthetic oils, such assynthetic hydrocarbon oil, ester oil, fluorine oil and silicone oil.From the viewpoint of heat resistance, fluorine oil is preferable, andexamples of the fluorine oil include perfluoropolyether oil and a lowpolymer of ethylene chloride trifluoride. The low polymer of ethylenechloride trifluoride may have a weight-average molecular weight of 500to 1200.

In the grease, other thickening agents may be further used incombination. Examples of the thickening agent include metallic soap,composite metallic soap, bentonite, phthalocyanine, silica gel, a ureacompound, a urea-urethane compound, a urethane compound, and an imidecompound. Examples of the metallic soap include sodium soap, calciumsoap, aluminum soap and lithium soap. Examples of the urea compound, theurea-urethane compound and the urethane compound include a diureacompound, a triurea compound, a tetraurea compound, other polyureacompounds, urea-urethane compounds and diurethane compounds, andmixtures thereof.

The grease preferably contains the powdery composition containing lowmolecular weight PTFE in an amount of 0.1 to 50 mass %, more preferablycontains the composition in an amount of 0.5 mass % or more, and morepreferably contains the composition in an amount of 30 mass % or less.Since the powdery composition containing low molecular weight PTFE iscontained in the above amount, a grease having an appropriate hardnesscan be obtained. The grease can exhibit sufficient lubricity and canexhibit appropriate sealing property.

The grease can further contain a solid lubricant, an extreme pressureagent, an antioxidant, an oiliness agent, a rust preventive, a viscosityindex improver, a detergent dispersant, etc.

EXAMPLES

The present invention will be more specifically described with referenceto the following Examples, but the present disclosure is in no waylimited to those Examples.

Comparative Example 1

In a barrier nylon bag, 50 g of POLYFLON (R) PTFE F-104 (manufactured byDAIKIN INDUSTRIES, LTD., concentration of PFC and concentration of PFOAwere each the detection limit or less) was weighed, and the bag wassealed by means of heat sealing. Subsequently, the PTFE F-104 in the bagwas irradiated with cobalt-60 γ-ray at 150 kGy at room temperature,thereby obtaining a low molecular weight PTFE powder. The resulting lowmolecular weight PTFE powder was used as a sample for measuring contentsof PFOA and a perfluorocarboxylic acid having 6 to 14 carbon atoms andits derivative. Further, a melt viscosity of the resulting low molecularweight PTFE powder was measured. The results are set forth in Table 2.

Examples 1 to 6

The low molecular weight PTFE obtained in Comparative Example 1 wasplaced in a sealable stainless steel container, and the container wasset in a state of reduced pressure. Thereafter, a process in which thecontainer was pressurized with nitrogen gas until the pressure in thecontainer became 0.5 MPa and then depressurized again was carried out 5times. Thereafter, the container was pressurized with nitrogen gas againuntil the pressure in the container became 0.1 MPa. Subsequently, theoxygen concentration in the container was analyzed by using gaschromatography, and the analysis demonstrated that the oxygenconcentration was 10 ppb or less. Next, the container was depressurizedagain, thereafter fluorine gas and nitrogen gas were injected into thecontainer until normal pressure was reached, and then the container wassealed. In order to accelerate fluorination reaction, the temperature ofthe stainless steel container was set at 200° C., and in an electricfurnace maintained at 190° C. to 210° C., the container was allowed tostand still for the time described in Table 2, thereby obtainingfluorination-treated low molecular weight PTFE. The concentrations ofthe fluorine gas injected into the container are as in Table 2.

(Measurement of Melt Viscosity)

Melt viscosity of the low molecular weight PTFE obtained in each ofExamples and Comparative Example was measured. In accordance with ASTM D1238, the melt viscosity was measured using 2 g of a sample having beenheated in advance at the measuring temperature (380° C.) for 5 minutesand using a flow tester (manufactured by Shimadzu Corporation) and a dieof 2ϕ-8 L while maintaining the sample at the above temperature under aload of 0.7 MPa.

(Measurement of Content of PFOA and Its Salt)

Using a liquid chromatograph mass spectrometer (Waters, LC-MS ACQUITYUPLC/TQD), a content of PFOA in the low molecular weight PTFE obtainedin each of Examples and Comparative Example, based on 100 mass parts ofthe low molecular weight PTFE, was measured.

Specifically, 5 ml of acetonitrile was added to 1 g of a powder formeasurement, and the mixture was subjected to ultrasonic treatment for60 minutes to extract PFOA. A content of PFOA in the resulting liquidphase was measured using MRM (Multiple Reaction Monitoring) method.Acetonitrile (A) and an ammonium acetate aqueous solution (20 mmol/L)(B) were used as mobile phases, and they were fed at a concentrationgradient (A/B=40/60−2 min−80/20−1 min). ACQUITY UPLC BEH C18 1.7 μm wasused as a separatory column, the column temperature was set at 40° C.,and the injection quantity was set to 5 μL. In the ionization method,ESI (Electrospray ionization) Negative was used, a cone voltage was setat 25 V, and measurement of precursor ion molecular weight/product ionmolecular weight resulted in 413/369. The content of PFOA was calculatedusing an external standard method. The detection limit in thismeasurement was 5 ppb.

(Measurement of Content of Perfluorocarboxylic Acid Having 6 to 14Carbon Atoms and Its Derivative)

Using a liquid chromatograph mass spectrometer (Waters, LC-MS ACQUITYUPLC/TQD), a content of a perfluorocarboxylic acid having 6 to 14 carbonatoms in the low molecular weight PTFE obtained in each of Examples andComparative Example, based on 100 mass parts of the low molecular weightPTFE, was measured.

Specifically, the above content was measured by the use of the MRMmethod using, as a solution, a liquid phase extracted in the measurementof PFOA. Under the measurement conditions changed in concentrationgradient (A/B=10/90−1.5 min−90/10−3.5 min) from the measurementconditions for PFOA, the precursor ion molecular weight/product ionmolecular weight described in Table 1 were measured. The total amount ofperfluorocarboxylic acids having 6 to 14 carbon atoms was calculatedfrom the following formula using the content (X) of perfluorooctanoicacid obtained in the above measurement. The detection limit in thismeasurement was 5 ppb.

(Ac₆+A_(c7)+A_(c8)+A_(c9)+A_(c10)+A_(c11)+A_(c12)+A_(c13)+A_(c14))/A_(c8)×X

wherein A_(c6) to A_(c14) represent peak areas of carboxylic acidshaving 6 carbon atoms to 14 carbon atoms, respectively; and

X represents a content of perfluorooctanoic acid (8 carbon atoms)calculated from the result of the measurement by the MFM method, usingan external standard method.

TABLE 1 Number of carbon atoms Precursor ion Product ion of carboxylicacid molecular weight molecular weight 6 313 269 7 363 319 8 413 369 9463 419 10 513 469 11 563 519 12 613 569 13 663 619 14 713 669

The results are shown in the following table. In the following table,the “fluorine gas concentration” indicates a volume ratio (%) offluorine gas to the total volume of fluorine gas and nitrogen gas in thegas used at the start of treatment in the step (II). In the followingtable, the “content of C6 to 14” indicates a content ofperfluorocarboxylic acids having 6 to 14 carbon atoms and theirderivatives.

TABLE 2 Step (II) Content Fluorine gas Content of C6 to Melt Raw Step(I) concentration Contact Contact of PFOA 14 viscosity materialTemperature Atmosphere Dose (vol %) method time (ppb) (ppb) (Pa · s)Comp. F-104 room air 150 kGy — — — 92 255 5.5 × 10⁴ Ex. 1 temperatureEx. 1 F-104 room air 150 kGy 0.1 200° C., 12 hours <5 <5 5.6 × 10⁴temperature sealing Ex. 2 F-104 room air 150 kGy 1 200° C., 6 hours <5<5 5.6 × 10⁴ temperature sealing Ex. 3 F-104 room air 150 kGy 1 200° C.,12 hours <5 <5 5.6 × 10⁴ temperature sealing Ex. 4 F-104 room air 150kGy 10 200° C., 6 hours <5 <5 5.6 × 10⁴ temperature sealing Ex. 5 F-104room air 150 kGy 10 200° C., 12 hours <5 <5 5.6 × 10⁴ temperaturesealing Ex. 6 F-104 room air 150 kGy 22 200° C., 12 hours <5 <5 5.6 ×10⁴ temperature sealing

INDUSTRIAL APPLICABILITY

The low molecular weight PTFE obtained in the production method of thepresent invention can be usefully used for producing plastics, inks,cosmetics, coating materials, greases, etc.

1-10. (canceled)
 11. A method for producing a composition containing lowmolecular weight polytetrafluoroethylene, comprising: (I) irradiating acomposition containing high molecular weight polytetrafluoroethylenewith ionizing radiation to obtain a composition containing low molecularweight polytetrafluoroethylene having a melt viscosity at 380° C. in therange of 1.0×10² to 7.0×10⁵Pa·s; and (II) carrying out fluorinationtreatment on the composition containing low molecular weightpolytetrafluoroethylene obtained above.
 12. The method for producing acomposition containing low molecular weight polytetrafluoroethyleneaccording to claim 11, wherein the step (II) comprises carrying outfluorination treatment and carrying out grinding treatment on thecomposition containing low molecular weight polytetrafluoroethyleneobtained in the step (I).
 13. The method for producing a compositioncontaining low molecular weight polytetrafluoroethylene according toclaim 11, wherein the step (II) comprises carrying out grindingtreatment on the composition containing low molecular weightpolytetrafluoroethylene obtained in the step (I) and thereafter carryingout fluorination treatment.
 14. The method for producing a compositioncontaining low molecular weight polytetrafluoroethylene according toclaim 11, wherein the fluorination treatment is carried out by bringingthe composition containing low molecular weight polytetrafluoroethyleneobtained in the step (1) and at least a fluorinating agent into contactwith each other.
 15. The method for producing a composition containinglow molecular weight polytetrafluoroethylene according to claim 14,wherein the fluorinating agent is a nucleophilic fluorinating agent oran electrophilic fluorinating agent.
 16. The method for producing acomposition containing low molecular weight polytetrafluoroethyleneaccording to claim 14, wherein the fluorinating agent is fluorine gas.17. The method for producing a composition containing low molecularweight polytetrafluoroethylene according to claim 11, wherein thecomposition containing high molecular weight polytetrafluoroethylene ispowder.
 18. The method for producing a composition containing lowmolecular weight polytetrafluoroethylene according to claim 11, whereinthe composition containing low molecular weight polytetrafluoroethyleneobtained in the step (I) further contains a low molecular weightfluorine-containing compound having a functional group; and the lowmolecular weight fluorine-containing compound having a functional groupis an acid having a fluorine-containing carbon chain having 6 to 14carbon atoms or a derivative thereof.
 19. The method for producing acomposition containing low molecular weight polytetrafluoroethyleneaccording to claim 18, wherein a content of the acid having afluorine-containing carbon chain having 6 to 14 carbon atoms or aderivative thereof in the composition containing low molecular weightpolytetrafluoroethylene obtained in the step (II) is 50 mass ppb or lessbased on the low molecular weight polytetrafluoroethylene.
 20. Themethod for producing a composition containing low molecular weightpolytetrafluoroethylene according to claim 18, wherein the acid having afluorine-containing carbon chain having 6 to 14 carbon atoms or aderivative thereof contains perfluorooctanoic acid or a salt thereof;and a content of the perfluorooctanoic acid and a salt thereof in thecomposition containing low molecular weight polytetrafluoroethyleneobtained in the step (II) is less than 25 mass ppb based on the lowmolecular weight polytetrafluoroethylene.